Summary

The Shivering Isles are just what Oblivion needed

The Good

To start things off I want to be honest:
I didn't expect anything - but got more than I could hope for. After charging money for several more or less disappointing "low content" add-ons and one bigger retail package which story didn't interest me much, Bethesda finally made an expansion worth the description.

Because Shivering Isles doesn't add anything to the world of Cyrodiil itself (except the gate to Sheogorath's realm, of course), the new world could been made rather extensive. The landmass might be about one fifth to one fourth of Oblivion's, there are several new dungeons, camps, villages and other sights to explore.
What really distincts these new places and their inhabitants from the known in the original game is a completely new design matching the Madgod's mind. The northern half of the Isles is overly bright and colorful, while the southern half appears dark and gloomy, each an expression of Sheogorath's mental aspects of Mania and Dementia. This separation affects both the Isles' creatures and people, so you encounter every facet of creative lunacy, paranoia, phobia and weird fetish you might think of.

The new styles are well executioned regarding graphics and plot-line, the latter being especially concentrated in the capital New Sheoth, where you will find most of the expansion's merchants and quest-giving NPCs. Of course, every single one of them is insane in his or her own way and that's what makes them really special characters who you will surely remember and maybe even start to like (I've kind of grown attached to the blacksmith Cutter, although the girl regularly gives me the creeps).

In traditional Elder Scrolls fashion you're able to ignore the main story completely, but you would miss one of the expansion's real strengths:
Aiding Sheogorath in the defense of his realm against an upcoming threat reveals a plot full of little twists and turns, sometimes forcing the player to make rather difficult decisions regarding his further way of action. It's exactly these things and the slightly mysterious background I loved in Morrowind and felt a bit lacking in Oblivion until now.

Another highlight is the new equipment, including two complete armor sets, various unique items and a bunch of new weapons. After using the same old high-level gear for maybe more than 30 or 40 hours worth of gameplay I was more than glad to see that Shivering Isles offers actual alternatives to glass or daedric armor and weapons, some of them noticeably more powerful than anything you are able to find in Cyrodiil.

Still, the new monsters and enemies remain challenging, but not too frustrating - even for a stealthy character not highly skilled in combat and magic - and all of them are exclusive to the Daedra Prince's realm. Gamers who played Morrowind before will have one or two déjà vus, though.
No worries, I am not talking about Cliff Racers here.

Depending on how much adventuring you're going to do you get approximately 15-20 hours of playtime for the main quest alone and another minimum of 20-30 hours if you're planning to complete all the side quests and explore the whole island. A pretty good deal for an add-on considering many complete games don't offer that much today.

The Bad

When I think about it, I cannot recall a lot of negative aspects of the expansion. One central point you could criticize already exists in the main game:
Although the explorable dungeons got various whole new designs fitting the Isles, they tend to feel a bit repetitive if you visit too many of them in a row. This is unfortunately supported by the fact that Sheogorath assigns you to a couple of "go to dungeon X and get Y"-quests later in the game, leading to a lot of dungeon-crawling. It's by far not as bad as closing a dozen Oblivion gates, though.

The way of travelling between Cyrodiil and the Shivering Isles can be a bit bothersome, too, if you - like me - collect "anything that's shiny at least once" or need to get equipment from one world to the other frequently. Because you have no possibility to fast-travel between the realms, you always have to travel to the gate first, then through the gate and finally to your original destination. Which really might be more a problem of convenience than actual gameplay, but a chance to switch the maps and directly fast-travel to the other world would have been a nice solution.

However, the expansion seems to have introduced one or more bugs which apparently led to occasional game crashes I hadn't experienced in Oblivion before. Also, there is a more serious bug in the expansion that may corrupt savegames after about 50-100 hours of playing. Up to the time I'm writing this (04/27/2007) there is only an official emergency beta patch or an user-made plug-in available to prevent this from happening.
Except those two the game didn't show me any obviously glitches.

The Bottom Line

Shivering Isles is a more than solid and beautiful-looking addition to Oblivion, especially for anyone who felt Cyrodiil was a tad *too* plain in terms of style and story. It looks like Bethesda allowed their designers and writers to play with a lot of crazy ideas to create an original and well made expansion. So if you liked the flair of Morrowind or the "A brush with death" quest in Oblivion, for example: grab it, it's worth the money.
However, Mania and Dementia are places of pure insanity and have their own definition of morale, which can be kind of twisted every now and then. So if you or your character are more of the saint-type of person, you might better want to stay away from the realm of the Daedra Prince of Madness.